Fisheries Biology, Assessment and Management

By Michael King

This excellent second edition of Fisheries Biology, Assessment and Management, has been fully updated and expanded, providing a book which is an essential purchase for students and scientists studying, working or researching in fisheries and aquatic sciences.

In the same way that excessive hunting on land has threatened terrestrial species, excessive fishing in the sea has reduced stocks of marine species to dangerously low levels. In addition, the ecosystems that support coastal marine species are threatened by habitat destruction, development and pollution. Open access policies and subsidised fishing are placing seafood in danger of becoming a scarce and very expensive commodity for which there is an insatiable demand.

Positive trends include actions being taken to decrease the incidental catches of non-target species, consumer preferences for seafood from sustainable fisheries, and the establishment of no-take areas that provide refuges for marine species. But there is an urgent need to do more.

Because there is an increasing recognition of the need to manage ecosystems as well as fish stocks, this second edition of this bestselling text book includes an additional chapter on marine ecology. Chapters on parameter estimation and stock assessment now include step-by-step instructions on building computer spreadsheet models, including simulations with random variations that realistically emulate the vagaries of nature. Sections on ecosystem management, co-management, community-based management and marine protected areas have been expanded to match the increased interest in these areas.

Containing many worked examples, computer programs and numerous high quality illustrations, Fisheries Biology, Assessment and Management, second edition, is a comprehensive and essential text for students worldwide studying fisheries, fish biology, aquatic and biological sciences. As well as serving as a core text for students, the book is a superb reference for fisheries and aquatic researchers, scientists and managers across the globe, in both temperate and tropical regions. Libraries in all universities where fish biology, fisheries, aquatic sciences and biological sciences are studied and taught will need copies of this most useful new edition on their shelves.

Supplementary material is available at: www.blackwellpublishing.com/king

• Language: English
• Publisher: Wiley
• Release date: Apr 16, 2013
• ISBN: 9781118688045

Book preview

1

Ecology and ecosystems

1.1 Introduction

Studies in biology, the science of living things, can be directed at increasing levels of biological organization from molecules, cells, organs, to organisms (or species) and beyond to populations and ecosystems. This chapter is concerned with higher levels of biological organization – populations, communities and ecosystems. A population is a group of individuals belonging to the same species and a community is a collection of populations inhabiting a particular area. An ecosystem is a functional and relatively self-contained system that includes communities and their nonliving environment.

Studies in fisheries biology have been directed at particular species targeted by fishing operations. Now, particularly in the light of decreasing catches and threatened marine ecosystems, there is a need for fisheries managers to take a broader view, one that includes the interdependence of target species, other species and the marine environment. The ecosystem is the basic unit of ecology, and can be defined as the study of the interactions between groups of organisms and their environment. The environment of an organism includes all external entities and factors that affect it, and therefore includes physical factors such as light, temperature and oxygen as well as other living things such as competitors, mates, predators, and parasites.

Although it is common, and often useful, to apply the term ‘ecosystem’ to particular entities, such as coral reef ecosystems or estuarine ecosystems, it must be realized that these are not isolated units. Ecosystems are linked to one another by biological and physical processes. In marine ecosystems, these linking processes include biological factors, such as migration and food chains, as well as physical ones, such as ocean currents and tides. Pursuing these linking factors, it becomes apparent that the entire planet can be regarded as an ecosystem, and is sometimes referred to as a biosphere. However, a more restricted view of an ecosystem – as the plants, animals and environment of a particular type of habitat, such as a coral reef – provides a more manageable entity for study and management.

1.2 Distribution and abundance

Populations are groups of individuals belonging to the same species. In fisheries biology, the word ‘stock’ is sometimes used interchangeably (and loosely) with ‘population’. In the strict sense, a stock is a distinct, reproductively isolated population which exists within a defined spatial range.

1.2.1 Unit stocks

Fisheries studies and management are concerned with a unit stock, which may be defined as a discrete group of individuals that has the same gene pool, is self-perpetuating, and has little connection with adjacent groups of the same species. Although this definition may not satisfy biogeographers, it does describe a unit which, because it has similar biological characteristics, may be studied, assessed and managed as a discrete entity.

Some species exist within a wide geographic range as a collection of unit stocks. The cod, Gadus morhua, for example, is distributed across the North Atlantic (Fig. 1.1) and within this relatively large range, exists in more or less isolated subpopulations or races. In such cases, fishing on one subpopulation appears to have no effect on others.

Fig. 1.1 The distribution of the cod, Gadus morhua, in the North Atlantic.

The boundaries of a unit stock are often difficult to determine, and many seemingly isolated populations may receive new recruits from other distant reproducing populations. Even in stocks of fishes on isolated reefs, the ability of larvae to drift and survive for a considerable time in the plankton allows them to reach other reefs some distance away. For example, the snapper, Lutjanus kasmira, which was deliberately introduced into Hawaii, spread throughout all the reefs and islands of the archipelago within a period of ten years (Oda & Parrish, 1981). Non-migratory species that live in widely separated areas, such as seamounts, must either rely on larval drift to replenish their populations or, if their larval lifespan is short, be self-sustaining.

From a fisheries assessment and management viewpoint, it is important to determine whether two adjacent stocks are either sufficiently interactive to be regarded as a single unit stock, or independent enough to be treated as separate unit stocks. In most cases, several criteria are used to confirm or refute a stock’s unit status. The penaeid prawn, Penaeus latisulcatus, for example, is caught by trawlers in two adjacent gulfs in South Australia (Fig. 1.2) but not in the area of sea between the two gulfs. As each gulf contains mature individuals and has coastal mangrove areas where juveniles are found, the stock in each gulf has the ability for self-replenishment. In addition, as tagging studies have not revealed any migration of prawns between the two areas, the stocks in each of the two gulfs are regarded as two separate units for research and management purposes.

Fig. 1.2 Two separate unit stocks (shaded areas) of the penaeid prawn, Penaeus latisulcatus, in two gulfs in South Australia.

1.2.2 Spacing of organisms

Within a unit stock, individuals may be distributed uniformly, randomly or in aggregations (Fig. 1.3). A uniform or even distribution rarely occurs in nature, mainly because the environment is rarely uniform. Even if the environment is relatively even, such as on a sandy sea floor, the distribution of sedentary species is likely to be non-uniform as a result of the uneven settlement of larvae from the plankton. However, a uniform distribution may be approximated in species where there is competition, territoriality or aggression between individuals. Territorial reef fishes, for example, often exclude others of the same species from a range around a home base on the reef. Random distributions are also rare in nature, if only because the aspects of the environment on which the species depend, such as food and shelter, are not randomly distributed.

Fig. 1.3 Types of spacing of individuals within an area.

Although widely-spaced individuals avoid intraspecific competition, this is often at the expense of advantages which may accrue to those living in aggregations, groups, or schools, the most common type of distribution. The advantages of living in aggregations in mating species may include better access to sexual partners, and in broadcast spawners, an enhanced confluence of eggs and sperm. Aggregations may also provide an increased ability to locate food and a degree of protection from predators. For example, among aggregating sea urchins, fertilization success is high, the trapping of drift algae for food is enhanced, and the spine canopy of the aggregation is a formidable deterrent to predators. Whatever the spacing, the overall distribution of individuals or clumps will be influenced by differences or gradients in the environment. In all marine organisms, a differential distribution with depth is to be expected, and most species occur in maximum numbers over a relatively narrow optimal depth range.

In fisheries studies the estimation of abundance, or at least relative abundance (the number of individuals at one time relative to the number present at another time), is important in determining the effects of fishing and environmental disturbances. Methods of estimating abundance are presented in Section 4.2.

1.3 Population growth and regulation

Populations of all organisms fluctuate around a mean level as long as deaths are balanced by births. In cases of populations, such as many fish stocks, which are overexploited or threatened by environmental degradation, deaths will exceed births and numbers will decrease. When an exotic species is introduced into a ‘new’ and suitable environment (Section 1.5.4 ‘Species invasions, introductions and translocations’) its population will increase, often in the absence of predators, until it is contained by the lack of food or living space. This section provides an introduction to populations as well as the factors that affect and regulate them.

1.3.1 Population growth

In the absence of limiting factors in conditions of unlimited food and living space, the increase of numbers in a population would be immense. For example, if a female shrimp produced 50 thousand female larvae, her descendants could number 2500 million females after just one additional annual spawning event as long as all the resulting larvae survived to reproduce. If N is the number of individuals in the population at a particular time, b is the birth rate, and d is the death rate, the population growth rate (I) is:

(1.1)

It is the value of (b − d), referred to as the intrinsic rate of population increase, that determines whether a population will decrease, remain stable (at zero population growth) or increase. As long as the average birth rate, b, exceeds the average death rate, d, the population (N) will increase. In addition, if populations are increasing, N will become larger with each generation causing the rate of increase to rise further. This multiplying rate of increase causes population numbers to increase as shown in the left-hand curve in Fig. 1.4: the curve becomes steeper and steeper until the population is expanding at an infinite rate. All organisms have such potential for exponential growth in the absence of any limiting environmental factors. But, as the world is not packed full of shrimps or any other organism, it is obvious that the increase in numbers in real populations is being held in check, or regulated, by one or more factors.

Fig. 1.4 Population growth curves, (from the left) without limits, with density-dependent limits and with density-independent limits.

1.3.2 Population regulation

All populations are limited in abundance by their requirement for resources – for essentials such as food and living space. Competition for these resources, and predation, cause the rate of population growth to decrease at high densities. These limiting factors are regarded as density-dependent because their effects increase as population density increases – for example, the effects of shortages of food and living space (starvation and crowding) increase with population size. Over time, population numbers follow an S-shaped curve in which an initial increasing rate of growth is followed by a decreasing rate as the curve approaches an asymptote (at what is known as the carrying capacity of the environment) imposed by one or more limiting factors (middle curve in Fig. 1.4).

Although populations of all organisms have limits to their growth, the human population is increasing, seemingly without limit. The carrying capacity of the planet in relation to human food has been increased by highly productive agricultural systems (although the distribution of food is inequitable) and more stringent social controls have allowed people to live in more crowded living spaces. How long this high rate of human increase can continue is not known. But leaving quality of life and aesthetic considerations aside, each additional person contributes to the planet’s environmental ills and the demand for natural resources. The overexploitation of fisheries resources and degradation of the environment are attributable to the rapidly increasing human population.

The numbers in some populations vary around the carrying capacity of the environment but in others, numbers fluctuate widely in response to factors that are unrelated to population density. The classic terrestrial example of this is the fluctuating population of some insects that appear in large numbers in warm humid conditions and disappear as the weather becomes cooler and drier. In this case, the weather is imposing a density-independent limitation on population size – that is, the weather is unrelated to (is independent of) population density.

Some species of shrimps or prawns, often regarded as the insects of the sea, also have populations that vary greatly from year to year depending on conditions in shallow nursery areas. Populations of some species of fish thrive in brackish-water estuaries during ideal conditions but die in large numbers with influxes of freshwater during heavy rain and floods. Populations of molluscs living in shallow tidal pools may suffer high mortalities during extended periods of low tides and hot weather when water temperatures climb and the amount of dissolved oxygen decreases. Storms and human interference, including shoreline building projects, produce silt that reduces the amount of sunlight reaching light-dependent organisms, such as algae, giant clams and coral. In these examples, salinity, temperature, dissolved oxygen and subsurface light are imposing density-independent limits on populations.

1.3.3 Life history patterns

The evolution of a particular life-history pattern in a species, including specific growth rates, mortality rates, and reproductive strategies, depends on a complex array of selective forces imposed on a species by its environment. Stock numbers are a result of the recruitment rate, birth rate and mortality rate of the species and are under the control of density-dependent and densityindependent effects.

In stable or predictable environments, species are more likely to be under the control of densitydependent effects, such as competition for food and space, and stock sizes may be relatively constant over time. In more variable environments, density-independent effects, such as extreme water temperatures, storms, and adverse currents, are likely to result in stock numbers fluctuating over time; any deviation away from optimal conditions will result in a decrease in numbers, and any subsequent improvement in conditions will be followed by an increase in numbers.

Many fish stocks appear to exist in numbers less than the carrying capacity of their environment and occasional large recruitments produce unusually large cohorts in many commercial fish stocks. The major part of the catch in some fisheries on long-lived species is often based on one or two large year classes which dominate and progress through length–frequency distributions over several years. The fact that these occasional strong year classes persist in adult fish stocks over many years suggests that the additional production is within the carrying capacity of the environment.

Classical theory relating the life histories of species to the physical and biological nature of their environment has been based on MacArthur and Wilson’s concept of r- and K-selection (MacArthur & Wilson, 1967). In this concept, r denotes the intrinsic (unlimited) rate of increase in population size, and K is the environmental carrying capacity, or the upper limit of numbers which can be supported by the environment. Species may be classified by the life-history characteristics which control their populations. According to one theory (Pianka, 1970), r-selected species have the ability to increase rapidly in number to take advantage of temporarily favourable environments, and K-selected species inhabit stable environments where the ability to compete with rival species is more important (Table 1.1).

If a species lives in an unpredictable or variable environment, where the chances of survival are uncertain, evolution is likely to favour early maturity and a single massive reproductive event (semelparity). In this case, spawning early in life, in the face of poor survival prospects ensures the success of the species. Success, from a species’ rather than an individual’s viewpoint, is measured in terms of its ability to perpetuate itself – i.e. reproductive success. Growth in such species is likely to be rapid, mortality is often density-independent and high, and the lifespan will be short (see Box 1.1 ‘Live fast, die young’).

If a species lives in a more constant or benign environment, however, the ability to reproduce many times (iteroparity) over an extended lifespan may represent the most successful life-history strategy. Reproductive success, therefore, is the result of a balance between the energy and matter devoted to reproduction, and that devoted to the maintenance and growth of the parent. Whether a greater or lesser proportion of available resources is devoted to reproduction or the well-being of the parent in a particular species, depends on the species’ environment.

The attractiveness and oversimplification of the r–K theory has led many researchers to label a given species as either r-selected or K-selected, when, at best, a species may be placed somewhere along an r–K continuum based on its particular life-history characteristics. In addition, the uncritical application of competition-based theory is hazardous because little is known of the intensity of competition in different environments. An example of a study of various related species distributed at different depths (and therefore subject to different environmental conditions) is given in Box 1.2 ‘Life history patterns and depth’.

Table 1.1 Correlates of r- and K-selection. Adapted from Pianka (1970).

Box 1.1 Live fast, die young

The vertebrate with the shortest recorded lifespan may be the coral reef pygmy goby, Eviota sigillata, which lives for just eight weeks and grows to less than 20 mm in length (Depczynski & Bellwood, 2005). Coral reef pygmy gobies spend their first three weeks as larvae in the open ocean before undergoing metamorphosis and returning to settle on the reef, where they mature within one to two weeks and have just three weeks in which to reproduce and contribute to the next generation. Although larval mortality in reef fishes is typically high, and the small body size of the goby limits the number of eggs produced, pygmy gobies are very successful and are found across the Indian and Pacific oceans. The key to the species’ success may be parental care: adult males fan and guard the eggs until hatching.

When the chances of survival are small, evolution often favours a ‘live fast, die young’ strategy (r-selection in Table 1.1) in which rapid growth and maturation offset the reduced chances of survival. However, rather than producing a single and massive quantity of small eggs, the pygmy goby produces a smaller number of eggs (about 400 eggs in three clutches) and invests energy in protecting them. This reproductive strategy, which enhances the survival of offspring by egg protection, is found in a few other fish species and is common in gastropod molluscs, some of which produce elaborate cases to protect a few large eggs (see Chapter 2).

Box 1.2 Life-history patterns and depth

Caridean shrimps (Fig. B1.2.1) are distributed in different but often overlapping depths on the outer reef slopes of tropical Pacific islands and have been the subject of a study on the relationship of life-history patterns to depth (King & Butler, 1985). Species of caridean shrimps in shallower depths are exposed to a greater number of predators, and a more fluctuating environment than those in deeper water. Consistent with the predictions of r–K theory, species in shallower depths have higher growth rates, smaller and more eggs, earlier maturity, and shorter lifespans than deeper-water species. However, some of the findings were counter to competition-based ecological theory, and it was proposed that the decreased predation rates in deeper water allow deeper-water species to have an extended lifespan, an increased degree of iteroparity, and hence a corresponding increase in reproductive effort summed over the lifespan. As the planktonic larvae of caridean shrimps migrate through the height of the water column, the probability of larval survival is likely to decrease with increasing distance traversed, and with increasing time spent in the water body. This decrease in the probability of larval survival with increasing depth may be offset by the production of larger eggs by deeper-water species. Larvae hatching from larger eggs are often thought to have better survival prospects than those from small eggs; this may be due to an increased ability of larger larvae to withstand starvation, and to escape predation, or a shorter larval life and hence a shorter period of predation by planktivores.

Fig. B1.2.1 The generalized depth distribution of caridean shrimps on the outer reef slopes of tropical Pacific islands. From shallow to deep water, species increasingly have slower growth, later maturity and longer lifespans. From King (1987).

An alternative theory involves ‘bet-hedging’ (Stearns, 1976), where the benefits of high fecundity are believed to be offset by an increased adult mortality. This assumes that the energetic costs of reproduction are high, and any increase in fecundity is accompanied by an increased parental mortality. The production of offspring in any one event is therefore a ‘trade-off’ against the expectation of future offspring production. When the survival of pre-recruits is considered rather than that of adults, the traits predicted for fluctuating and stable environments may oppose those predicted by the r–K theory. For example, in environments where larval survival is variable, selection should favour reproductive patterns featuring an extended lifespan and iteroparity (Murphy, 1968). In this case, the repeated production of smaller broods decreases the chances of failure during a single reproductive event.

Considering the complex array of life-history characteristics that contribute to a species’ fitness in terms of survival, it is unlikely that any single ecological theory will adequately account for the evolved life-history patterns of all species. Nevertheless, concepts such as r–K theory provide useful clues to the likely range of life-history characteristics of species encountered in a particular environment.

Fisheries resource species may be classified according to their position on a continuum from those with a high natural mortality rate and a high reproductive capacity, to those with a low natural mortality rate and a low reproductive capacity. Short-lived species with high fecundity include shrimps, prawns, squid, and clupeoids, and long-lived species with low fecundity include sharks and many deeper-water and cold-water fish.

Short-lived species with high fecundity have the capacity to produce large numbers of prerecruits when environmental conditions are suitable. Sardines, for example, may take advantage of the increased productivity in an intermittent upwelling, and juvenile penaeid prawns may benefit from occasional ideal conditions in a nursery area. Such species are subject to large fluctuations in recruitment, and therefore stock size from year to year; a corollary of this is that the relationship between recruitment and stock size is usually poor. If short-lived species with high fecundity are overfished, recruitment levels may remain high and variable even at low stock levels. As recruitment varies greatly due to environmental effects, decreases related to low stock size may be hard to detect and recruitment may fail without warning.

Long-lived species with low fecundity, on the other hand, produce a smaller and more constant number of pre-recruits, which have relatively high probabilities of survival. Deeper-water fish, for example, live in a relatively constant environment, and unexploited species are likely to produce similar numbers of pre-recruits from year to year. Here there is likely to be a much stronger relationship between stock size and recruitment, and, if the species is fished, recruitment and catch rates will decrease immediately.

Life histories can be affected quite rapidly by exploitation. Phenotypic and genetic life-history responses to fishing can include increased individual growth rates, reductions in age and size at maturity, and genetic changes due to the high mortality rates on particular parts of the population. These changes can be rapid (in less than one generation) or slower in the case of selection for individuals that are less susceptible to fishing (Hutchings, 2002).

Life-history patterns and diversity also vary along environmental gradients other than depth (see Box 1.2). The most obvious trends in the number of species are the decreasing diversity with increasing latitude (from the equator to the poles) and with increasing distance from what are believed to centres of origin and speciation.

In fish species there is a significant negative trend from high diversity at the equator to lower diversity in higher latitudes but such is the variation within various fish assemblages in all latitudes that latitude change accounts for only 15% of the variance in available data (Martha et al., 2002). Diversity is known to decrease with increasing distances away from centres of high diversity but the reasons for this are unclear. Centre of origin hypotheses have been subject to criticism and differences in species diversity have been attributed to allopatric speciation models that include plate tectonics, sea-level fluctuations and island age (Mooi & Gill, 2002). The trend of decreasing diversity of scleractinian corals from the Western to the Eastern Pacific with increasing distance from the centre of biological diversity for the Indo-Pacific in Indonesia is shown in Table 1.2.

Table 1.2 Maximum number of reef-building coral genera by area in the South Pacific. Veron (1986).

With the decrease in diversity away from the Western Pacific there is a general increase in endemism. In spite of a low diversity of corals, Hawaii has a greater number of endemic species than other areas to the west (Veron, 1986).

1.4 Marine ecosystems

A small intertidal rock pool and a large body of water such as the entire Mediterranean Sea could each be described as an ecosystem – a relatively self-contained, functional system that includes communities and their non-living environment.

However, from a practical management viewpoint, an ecosystem is usually defined as some unit that can be studied, monitored and managed within the limits of available financial and human resources. Whereas managing a small rock pool is hardly worthwhile, managing an entire ocean is usually impractical. In most cases, managers will concentrate their efforts on a defined ecosystem that is large enough to encapsulate as many trophically dependent species and linked habitats as possible, but not so large that management (including monitoring and enforcement) is ineffective. Whether small or large, however, ecosystems are not independent of each other and boundaries placed between them are largely artificial.

Although, in most cases, ecosystems will be defined in terms of smaller manageable areas, some large geographical areas have been designated as ecosystems for management purposes. This has been done on the grounds that, although the areas contain a variety of habitat types, these are linked by ecological processes including currents and food chains. The Great Barrier Reef, for example, which stretches along 2000 km of the Australian coast, is managed by a single national authority.

Many larger marine ecosystems straddle political borders, necessitating the involvement of international and regional agencies in the management of shared resources and environments. Thus, for the purposes of monitoring and management, extensive regions of the sea have been identified as large marine ecosystems, or LMEs. These are large regions of ocean (of 2 000 000 km² or more) that share similar hydrographic characteristics and contain trophically dependent populations of aquatic species. LMEs can include semi-enclosed areas such as the Mediterranean and the Black Sea, continental shelf areas such as the Northwest Australian Shelf and coastal current systems such as the Benguela Current off the coast of south-western Africa. A FAO project on LMEs, launched after the 1992 Earth Summit (UNCED, Rio de Janeiro, 1992) has the primary objective of improving the long-term sustainability of international and coastal aquatic resources and environments. The project promotes the integrated management of coastal areas and the marine environment in order to halt or even reverse their deterioration.

Fig. 1.5 Common saltmarsh plants. Cord grasses, Spartina; glassworts, Salicornia; and rushes, Juncus. Drawing courtesy of Jeremy King.

A basic knowledge of the ecosystems on which marine species depend is required by those responsible for managing fish stocks and their habitats. This section provides an introduction under the three broad headings of: coastal waters; coral reefs and lagoons; and continental shelves and the open sea. These groupings could be said to include many separate ecosystems, and certainly many different habitats. More detailed ecological information can be gained from a good standard text such as Nybakken and Bertness (2005).

1.4.1 Coastal waters

Coastal zones are the interface between terrestrial and marine environments. At this interface, marine ecosystems receive nutrients from terrestrial sources via freshwater runoff, rivers and the scouring effect of high tides. The high productivity and accessibility of many coastal areas result in them being the most heavily fished areas of the sea.

In many coastal areas, freshwater and marine environments meet in wetlands and estuaries. In temperate areas, typical wetlands include salt marshes dominated by plants such as the cord grasses (Spartina), rushes (Juncus) and samphires or glassworts (Salicornia) shown in Fig. 1.5. A few organisms, including species of molluscs, crabs and smaller crustaceans, can tolerate the anoxic and highly saline substrates in salt marshes but much of the productivity is unused. Nutrients not used by the few resident saltmarsh organisms are transported via tides and runoff to enrich nearby estuaries and coastal areas.

Estuaries, areas at the mouths of creeks or rivers where freshwater and seawater meet and mix, are particularly rich in minerals and organic material. These semi-enclosed areas of brackish water with connections to the open sea are accessible to marine organisms that can tolerate varying conditions of salinity, temperature and turbidity. In much of an estuary, however, restricted light penetration and muddy substrates inhibit the presence of attached aquatic plants.

The usual widening of estuaries as they approach the sea results in the flow of water slowing down and releasing its load of lighter particles. These fine particles settle out of the water to form large banks of silt and mud where only a few species of algae can get a foothold, usually by attaching to shells and grit. However, diatoms and bacteria benefit from the high nutrient levels and make estuaries some of the most productive of all marine ecosystems. Estuaries often support large numbers of euryhaline, or salt tolerant species including many molluscs and worms that feed on the nutrient-rich deposits and, in turn, provide food for larger animals including fish. Nevertheless, estuaries usually have a lower species diversity than adjacent coasts as fewer organisms can tolerate the fluctuating environmental conditions.

The juveniles of many species, including shrimps, menhaden, anchovies, and mullets, grow in estuaries before migrating out to sea to breed. Although relatively few fish species are permanent residents of estuaries, many larger species including snappers, trevallies (jacks) and sharks periodically move into the lower reaches of estuaries to feed.

Some migratory fishes pass through estuaries to breeding grounds either in salt water or freshwater. Those that spend most of their lives at sea but move into rivers to release their eggs are known as anadromous species and the best known examples are the various species of salmon (family Salmonidae). Those that grow in rivers and move out to sea to breed are catadromous species and include freshwater eels of the genus Anguilla, that migrate out to places such as the Sargasso sea to spawn (see Section 2.3 ‘Fishes’).

In subtropical and tropical regions the most notable salt tolerant plants of estuaries are the 80 or so species of specialized, but often unrelated, trees collectively known as mangroves. Most mangroves grow where the water is brackish – either in estuaries or on coasts where seawater is diluted by freshwater soaks or runoff from the land. Mangroves have several special adaptations to equip them for living in silty, waterlogged soils. Many species have evolved exposed (or aerial) root systems that support the tree mass and absorb oxygen. The orange mangrove has exposed knee roots, the red mangrove has long prop roots which grow down from the trunk, and the black mangrove has cable roots which extend over a large surface area and send up small peg-like aerial roots or pneumatophores (Fig. 1.6).

Several families of fish are commonly associated with mangroves including gobies, gudgeons, silver biddies, sardines, snappers, slip-mouths, puffers and mullets. But a much larger number of smaller and microscopic species, including diatoms and bacteria, takes advantage of the large amount of organic material associated with mangroves. Nutrients from terrestrial runoff are taken up by the mangroves, which, through leaf drop and decay, contribute to detrital food chains. It has been estimated that from 2–18.7 tonnes of leaf litter can be produced each year by 1 hectare of mangroves (various researchers in Saenger, 1994).

Fig. 1.6 Mangroves. Examples shown are the orange mangrove, Bruguiera, with knee roots, the red mangrove, Rhizophora, with prop roots, and the grey or black mangrove, Avicennia, with pneumatophores.

Although mangroves cover less than 0.1% of the global land surface, they appear to account for as much as a tenth of the dissolved organic carbon (DOC) that flows from land to the ocean. Studies in Brazil (Dittmar et al., 2006) suggest that the plants are one of the main sources of dissolved organic matter in the ocean. Net-like mangrove roots trap carbon-rich leaf litter from which dissolved organic matter is leached by tidal flow into coastal waters. However, mangrove foliage has declined by nearly half over the last few decades because of increasing coastal development and habitat damage. Organic matter dissolved in the world oceans contains a similar amount of carbon as atmospheric carbon dioxide and the balance between the two is part of the global carbon cycle that regulates atmospheric carbon dioxide and climate. It has been speculated that reductions in mangroves and dissolved organic carbon may threaten this delicate balance, with potential consequences for atmospheric composition and climate.

Mangroves also protect and extend shorelines. Their exposed roots dissipate the energy of waves and cause currents to slow down to release suspended material. As the siltation continues, there is an ecological succession in which the mangrove front advances towards the sea and plants that are less salt tolerant become established in the area behind the front. In many parts of the world, mangroves and wetlands are protected because of their recognized role in marine ecosystems, particularly as nursery areas for many marine species. Nevertheless, such areas are under constant threat from pollution and land reclamation (see Section 1.5.1 ‘Habitat modification and loss’).

In temperate and tropical areas, the lowest tide levels of estuaries, lagoons and sheltered coastal areas are habitats for aquatic plants collectively called seagrasses (see Box 1.3 ‘The invasion of the sea by flowering plants’). Seagrasses are not true grasses, but have some grass-like features including leaves attached to short erect stems, creeping horizontal rhizomes and root systems. In addition to reproducing vegetatively via their network of rhizomes, which spread beneath the sand to form vast beds resembling underwater meadows, seagrasses produce small flowers that are fertilized by pollen floating in the sea.

In contrast with marine algae, relatively few animals consume seagrasses directly as the cellulose that constitutes the bulk of the plant makes it indigestible to most herbivores. Large animals that do feed on seagrasses include the green turtle as well as the Indo-Pacific dugong and its Caribbean relative, the manatee. Many more species, however, graze on the more easily digested fine mat of algae growing as epiphytes on the seagrass blades. Unlike marine algae that take nutrients from the surrounding water, seagrasses absorb nutrients via their roots and therefore recycle material that would otherwise be trapped in the substrate. As seagrass leaves eventually decay, the detritus formed is used as a source of food by a much wider range of marine species. Besides being highly productive areas, seagrass beds provide sheltered nursery areas in which the larvae and juveniles of many marine species live and grow before moving elsewhere as adults.

Away from the mouths of rivers, and without the input of terrestrially-derived nutrients, coastal areas are generally less productive but have a greater species diversity. About 40% of the world’s coastline is fringed by sandy beaches. Sand is supplied to beaches from the erosion of cliffs and rocks, by rivers, from offshore sediments and is lost by being blown inland and washed offshore during storms. Sand is also moved from one beach to another by longshore currents created when waves strike the coast at an angle (Fig. 1.7). In many parts of the world, groynes have been built out at right angles to shorelines to obstruct the longshore transport of sand and encourage the build-up of sand; although sand is deposited on the up-current side of the groyne, it may be scoured away from the down-current side.

Beaches are made up of particles ranging in size from the fine powder of silica sand to the large, well-rounded pebbles and cobbles of shingle beaches. Many beaches in temperate climates have yellow sand derived from quartz (a mineral form of silica that crystallizes as hexagonal prisms). Darker sands may display colours due to small proportions of different minerals such as mica (layered silica), rutile (titanium dioxide), feldspar (aluminosilicates), and magnetite (iron oxide). Some coasts have striking black sand beaches formed by the wearing away of dark volcanic basalt. Besides minerals ground from rocks and terrigenous material brought to the ocean from rivers, sand often contains material of a biological origin including shell fragments and the white sand of tropical beaches may be entirely derived from calcareous algae and corals (see Box 1.4 ‘Bioerosion’). About one-third of all sea floor sediments and, in some places, beach sand consists of the remains of foraminiferans (see Section 1.6.2).

Box 1.3 The invasion of the sea by flowering plants

Flowering plants first appeared on this earth about 100 million years ago, when the dinosaurs were wandering on dusty, grass-less soil, and became dominant over all terrestrial plants. Presumably competition for space and nutrients provided the driving force for some flowering plants to invade the sea. This invasion resulted in the successful establishment of a small group of flowering plants, the seagrasses, in the shallow margins of the sea. There are only about 50 different species of seagrasses, but these have been so successful that they are found in shallow coastal areas and estuaries in temperate and tropical regions around the world (Fig. B1.3.1).

Seagrasses play an important role in the coastal processes of many tropical and temperate shores by stabilizing drifting sand and contributing to marine food chains, particularly by leaf loss. Seagrasses shed their leaves seasonally and during storms these may be washed up on beaches in massive quantities. Seagrass leaves used to be collected to make alkaline potassium compounds, such as potassium hydroxide. The old name for these compounds was potash, and the name came from the way it was originally obtained – by burning seagrasses, leaching the ashes, and then evaporating the solution in large iron pots.

Fig. B1.3.1 Seagrasses. Examples shown are spear grass, Halophila, and turtle grass, Thalassia.

Fig. 1.7 The build-up of sand against a solid jetty in a longshore current that moves from right to left.

Box 1.4 Bioerosion

Bioerosion is the breaking down of substrates, usually coral, by the actions of various organisms (referred to as bioeroders). Some sponges, bivalve molluscs, and worms are internal bioeroders and bore into, and live in, the coral structure. External bioeroders, including some fish and sea urchins, feed on the surface of the coral. Two important bioeroders of corals are the colourful parrotfishes (family Scaridae) and sea urchins (Fig. B1.4.1).

Parrotfishes have massive fused teeth with which they scrape coral to feed on algae and symbiotic zooxanthellae within the coral polyps. The fishes have to graze large quantities of coral to gain a small amount of organic material and they appear to be continually evacuating clouds of fine coral particles. As each adult parrotfish can produce about one tonne of particulate matter each year, their contribution to the sand of lagoons and tropical beaches is considerable. Some sea urchins, such as various species of Diadema, are among the most important invertebrate bioeroders on coral reefs around the world (Glynn, 1997).

Fig. B1.4.1 Common bioeroders of coral. Left, the sea urchin, Diadema, and right, a parrotfish (family Scaridae).

In many parts of the world, sandy beaches are under threat from sand mining and foreshore constructions which change the natural paths of sand replenishment. Sand is mined for use in the construction industry (in the making of concrete) and in the manufacture of glass, which is made by melting together silica sand (SiO2), limestone (CaCO3), and sodium salts at very high temperatures.

The number of intertidal organisms living on sandy beaches is related to sand particle size and wave action. Beaches with strong wave action usually have coarse sand, as lighter particles which remain in suspension, are carried away by the backwash of waves. Conversely, beaches with smaller waves and a less powerful backwash are likely to have finer sand. In general, beaches of coarse sand are less suitable habitats for intertidal species. Finer sand retains more interstitial water during low tides and many intertidal animals are dependent on this water to obtain dissolved oxygen and to prevent desiccation.

Substrate mobility, the movement of sand due to currents and wave action, also has a marked effect on the number of species inhabiting beaches. Shifting sand makes it difficult for plants to gain a foothold and for animals to burrow deep enough to avoid dislodgement. In addition, beaches with heavy surf generally have lower quantities of nutrients in the substrate as the backwash of waves tends to remove most organic material. These difficulties are exemplified to the extreme on a cobble beach, the formation of which depends on at least moderate wave action to prevent cobbles being covered by sediment. The impossibility of burrowing and attachment as well as the risk of being crushed by the rolling cobbles, makes a cobble beach one of the most formidable coastal habitats for marine organisms to colonize.

Although sheltered environments generally have a greater diversity of species, high energy coasts such as surf beaches support a smaller number of specially-adapted organisms. There are no plants on the mobile substrate and the beaches are populated by herbivores that can utilize phytoplankton, which is often concentrated by wave action. Users of the phytoplankton include suspension feeders typified by surf clams of the genus Donax, which are found on high energy beaches around the world. These clams have wedge-shaped shells and large spade-like feet that allow them to burrow quickly and deeply to avoid repeated dislodgement by waves. Small mole crabs of the family Hippidae have rounded bodies with short strong limbs which provide them with the same ability. The colonization of such harsh environments suggests that the hazards involved are offset by some advantages, which may include living in a habitat where there are fewer predators and competitors.

Even though beach slope, sand particle size and wave action are interrelated, coasts are often classified on the basis of their exposure to waves as either, low, medium or high energy. The classification is useful even though there is obviously a continuum from low energy coasts such as sheltered bays and lagoons to high energy coasts such as exposed surf beaches. In general, it is found that the diversity and abundance of species decreases from low energy to high energy coasts. In a study of 105 beaches around the world, Bally (in McLachlan, 1983) found that both the abundance and mean number of macrofauna species decreased with increasing exposure to waves (Table 1.3).

Above the high tide mark, the backshore of a beach is submerged only during storms and may contain one or more berms – ridges of sand and debris running parallel to the beach. The foreshore extends from the low tide mark to the high tide mark. This littoral or intertidal zone, which lies between the extremes of high and low tides, is the transitional area between terrestrial and marine ecosystems.

Table 1.3 Mean number of macrofauna species and abundance on beaches with different wave exposure. Mean data from 105 beaches worldwide. From Bally in McLachlan (1983).

Tides are influenced mainly by the moon and the sun and, in spite of variations caused by weather patterns, are more or less regular (see Box 1.5 ‘The sun, the moon and the tides’). Most coasts of the world have one or two high and low tides each 24 hours and this regularity has allowed many aquatic animals to evolve for life in the intertidal zone. Intertidal organisms are exposed to the air for some part of each day and have to contend with fluctuating temperatures, the possibility of desiccation and the inability to gain oxygen and food when not covered with water. In spite of these difficulties, many different species inhabit the intertidal zone. The advantages of doing so may include escaping swimming predators, at least for some of each day, and gaining food that is less abundant or not available below the low tide mark.

A large number of marine animals, including those that live below low tide, display rhythms in spawning and feeding behaviours that are related to tides and the lunar cycle. A well-documented example of a species in which spawning is influenced by the tides is the grunion, Leuresthes tenuis, a fish that spawns on beaches on the Pacific coast of the United States. On certain nights of the highest spring tides, grunions move to the upper beach where they lay eggs that hatch about one lunar month later, when the fry can use the high tides to swim out to sea.

Tides are also responsible for the passive movements of marine animals during their life cycles and some species with a limited ability to swim appear to use tides to migrate in and out of inshore nursery areas. The juveniles of some penaeid shrimps or prawns, for example, live in estuarine nursery areas where they burrow during the day and emerge to feed at night as they drift with the tide. At times of the year when flood tides are more common during the night, juvenile prawns make a net movement up the estuaries each night before burrowing during the day. However, at the time of the year when ebb tides begin to predominate over flood tides each night, juvenile prawns make a net movement seawards during each night and this time marks the period in which juvenile prawns move offshore to join adult stocks (Penn, 1975).

Many predatory fishes actively move inshore on flood tides to prey on burrowing animals that, after a prolonged period of exposure, leave their burrows to gain oxygen and food from the inflowing water. Bottom-feeding fishes including rays and skates move in with the tide to feed on clams, crabs and other small invertebrates that emerge from the substrate.

The ability to withstand the effects of exposure to air varies between species and this accounts for the marked patterns in the distribution of organisms at different intertidal levels on all coasts. This zonation in the intertidal zone is particularly noticeable on rocky shores, which are more densely populated and have a greater diversity of species than sandy shores. Animals that need to live on, or even attached to hard substrates include barnacles, bivalve molluscs such as oysters and mussels, gastropod molluscs such as limpets, and some soft-bodied species including anemones and sea squirts.

The littoral or intertidal zone lies between the extremes of low and high water at spring tides and is further divided into other zones based on a universal scheme proposed originally for temperate rocky shores (Stephenson & Stephenson, 1972). The scheme, which is a description of (rather than an explanation for) the zonation of organisms on rocky shores, appears essentially applicable to tropical shores as well as temperate ones. The midlittoral zone can be thought of as lying between the low and high tide marks of average tides – that is, half way between neap and spring tides. This zone is regularly exposed and submerged on most tidal cycles. Above the midlittoral zone is the supralittoral fringe, into which the tide reaches only during spring tides and above this a spray zone that is dampened by the spray of waves. Below the midlittoral zone is a infralittoral (or sublittoral) fringe that is exposed only during spring tides.

A generalized distribution of organisms on a rocky shore is shown in Fig. 1.8. Above the spring tide high water mark, where rocks are moistened by sea spray, grey or orange lichens and tar-like patches of cyanobacteria may be found and rock crabs (such as those of the family Grapsidae) dwell in damp crevices. The level of high water of spring tides is often marked by bands of dark periwinkles (family Littorinidae) or, more commonly on tropical shores, the lighter coloured nerites (Neritidae). Almost universally, there is a light coloured band of acorn barnacles (balanomorph crustaceans) and limpets (molluscs of the family Acmaeidae) close to the average high tide mark and the upper limit of barnacles marks the upper limit of the midlittoral zone. Species of oysters that can tightly close their shells to avoid desiccation when exposed are cemented to rocks around the mid-tide mark. Mussels, with their thinner shells, do less well at dealing with exposure and are often restricted to the lower tide zone where they form dense dark bands.

Box 1.5 The sun, the moon and the tides

The sea covering the earth is under the attractive forces of both the sun and the moon. The strength of these forces is related to the mass of the body and its distance from earth – force is directly proportional to mass but inversely proportional to the square of distance. In spite of the moon’s smaller mass, its proximity to earth results in its attractive forces being much stronger than the sun’s, and so provides the major influence on tides. In Fig. B1.5.1, the fine lines above the earth’s surface show the tidal bulges of water caused by the gravitational pull of both the moon (the lunar tide) and the sun (the solar tide). The effect of these bodies is to create a bulge of water on opposite sides of the earth. Thus two areas on the earth’s surface are directly under a bulge of water, and are experiencing high tides, at any one time. As the earth is rotating, each point on the earth’s surface should (and many places do) have twice-daily or semidiurnal tides – that is, two high tides and two low tides each lunar day of 24 h 50 min.

Fig. B1.5.1 Top and middle diagram: the water covering the earth (indicated by the fine lines above the earth’s surface) is under the attractive forces of both the sun and the moon which result in (a) spring tides and (b) neap tides. Bottom diagram (c) shows mixed tides on the Pacific coast of Australia over a lunar month.

Fig. B1.5.1a represents the full moon (when the moon, illuminated by the sun, is visible as a full sphere from earth) and the new moon (when the moon lies between the earth and the sun). In both cases, the pull of the moon and sun is working together and result in two large tidal bulges on opposite sides of the earth. These are the times of spring tides when the tidal range (the difference between the heights of high and low tide) is greatest. Fig. B1.5.1b represents the first and third quarters of the moon, when only half the moon is illuminated by the sun. In these cases, the pull of the moon and sun is working at right angles to each other. These are the times of neap tides when the tidal range is least. As the moon circles the earth each lunar month of about 29.5 days, there are two neap tides and two spring tides during this period.

Tides are modified by other factors such as the declination of the moon and the depth and size of ocean basins. This results in three basic tidal patterns in various parts of the world. Semidiurnal tides (two highs and two lows per day) occur in the Atlantic Ocean and the Indian Ocean. Diurnal tides (one high and one low per day) are common in shallow semi-enclosed seas such as the Gulf of Mexico and along the coast of Southeast Asia. Mixed tides, in which there are two high and two low tides of unequal height per day, are the most common type (shown at the bottom of Fig. B1.5.1). Tides and tide ranges vary due to the influence of coastal formations including headlands, bays and offshore islands. The world’s largest tide range is at the head of the Bay of Fundy where the spring tide range reaches 15 m. In some estuaries with such large tidal ranges, the flood (rising) tide is held back by the restricting effect of decreasing depths and narrowing rivers, so that it rushes in on top of the water flowing downstream as a tidal bore. These turbulent masses of water have an almost vertical wave front which may reach heights of over 3 m and rush in at the ‘speed of a galloping horse’ – about 22 km/hr.

Fig. 1.8 The distribution of some characteristic organisms on a rocky shore (top) and a sandy beach (bottom) in the intertidal zone. Not all organisms shown are necessarily found in the same area. HWS = high water spring tides; LWS = low water spring tides; HWN = high water neap tides; LWN = low water neap tides.

At the lower part of the midlittoral zone, animals such as sea squirts and anemones may be found with several species of algae, including Porphyra and Fucus, that can withstand exposure for limited periods. In temperate and cold waters, large kelps extend up just above the lowest of low tides (to the upper limit of the sublittoral fringe): these include the leathery oarweed, Laminaria in the northern hemisphere and similar species such as the strap-weed, Ecklonia, in